Summary Report No. 56

An analysis on landscape level was performed to investigate the growth of Scots pine stands (Pinus sylvestris L.) in the research area of
"Dübener Heide" under the impact of rapidly changing pollution loads and to evaluate their further development.

Central to the work was a spatial database, installed using a
geographical information system (GIS). This database enabled the
statistical analysis of relationships and correlations between the
growth of the pine stands, the influence of pollution impacts and the
natural site characteristics. The results of emission and immission
modelling gave detailed evaluation of the pollution load over the
investigated area.

The spatial information database was linked with a process-based
growth model called FORSANA. The resulting regional model was used to
simulate forest growth on stand level for variable time periods. The
plausibility of the simulation results of the model was checked for the
influence of different patterns of pollution using the data available
for the investigated area. Simulation runs were also made with these
data to estimate the further development of the pine stand under
various climate and emission scenarios.

The theoretical background of the work was provided by the results
of forest decline research, considering especially the hypothesis of SO2-effects, nitrogen saturation and acidification on tree
growth. For the analysis of cumulative as well as concentration effects
and for the testing of the forest growth model a time span of
approximately 20 years was used.

The pollution pattern in the research area showed a quite rapidly change from high SO2-immissions and high nitrogen inputs linked
with a good base cation supply, to a pattern with low SO2-immissions with ongoing high nitrogen loads and an increasing soil
acidification.

The results of the statistical analysis show the main cause of the
variation in growth rates in the research area was atmospheric SO2-concentration. At the beginning of the investigated
time span, the stands already showed different growth patterns
according to previous damage. These differences are closely related to
the average gradient of SO2-immissions. The strong decrease of atmospheric
SO2-concentrations
led a to decreasing load in all areas as well as to an equalisation of
the pollution gradient. Present values are slightly above levels
described in the literature as harmless for forest trees. The gradient
of SO2-concentrations
may not influence the further growth, if no effects occur resulting
from prior loads. However, because of the prior damage differences in
yield will still be recognised in the next decades.

The high immissions of fly ash, especially in the western part of
the research area, led to accumulation of base cations in the soils.
The acidifying effects on the sulphate immission caused by high SO2-concentrations,
was buffered by the alkaline component of the fly ash especially on the
areas near the emitter. As well as this chemical reaction, the
immission of fly ash also had an effect on the improvement of the base
cation supply to the trees. This also may have compensated for some
direct damage caused by the SO2 load.

Measures for the reduction of fly ash emissions during a constant high level of SO2-emissions
led to increased potential of acidification in the precipitation, and
subsequently to a (re)acidification in soils. This process already
started in the 1980th. The leaching of sulphate from soils in the
western part of the research area is mainly buffered by calcium, while
in the eastern part, pedogeneous aluminium is already mobilised. There
was however no indication of yield losses or damage in the investigated
pine stands.

Despite the strong decrease of sulphate immissions after 1989 it is
nontheless to be expected that the tendency of acidification in the
soils will continue. This is seen as a result of the ratio between the
acidifying and basic compounds in precipitation, and the soil sulphur
content which is still high. Furthermore, the combination of enhanced
nitrogen input and a lowered cation supply or a high concentration of
aluminium compounds in the soil solution, leads to a higher risk for
tree growth and forest stand stability in the subsequence of nutrient
imbalances and aluminium toxicity. These kinds of problems are expected
first in the eastern part of the area, at sites which presently are
nitrogen saturated but already suffer from depletion of base cations.

The combination of strongly reduced SO2-immissions and
high nitrogen input led to a strong increase in growth rates. This is
why yields in the research area are somewhat higher than those in the
other parts of eastern Germany. High nitrogen inputs resulted also in
high eutrophication of forest soils. Subsequently a substantial amount
of the soil in the research area is extensively saturated with
nitrogen. Nitrogen immission rates are still high. Reductions are not
forseen at least not in the near future. For that reason a further
increase in the nitrogen content of the soils must be assumed. There
are first indications of humus disintegration and subsequent nitrogen
release from soils at several sites. The most probable effect of this
process would be increased nitrogen leaching into the groundwater. The
direct negative effect of enhanced nitrogen concentrations in soil on
the growth rate of the stands is related to the base cation supply, and
therewith to the development of soil acidifiation. At present, the
eutrophication gradient overlies the previous gradient of SO2-impact.

As well as nitrogen originating from traffic and other sources the
further development of agriculture close to the investigated area will
be of essential significance in the research area especially at its
margin sections. An important measure for the maintenance of the
nitrogen capacity and the protection of base cation levels in soils, is
the introduction of deciduous species into the pure pine stands.

Concerning the relationship between growth rate and site conditions,
a correlation could be shown between yield and the potential exchange
capacity for base cations in older stands only. Because of small
differences in soil status in the research area, significant effects on
yields can only recognised over several decades. Furthermore there is
evidence that, because growth rates are enhanced by nitrogen input and
SO2-reduction site characteristics relevant for the
maintenance of the water cycle become increasingly the limiting factor
of growth, especially for younger stands and may be the main driving
force for the differentiation of yield in future.

Problems for estimation of further development in the research area
could rise from uncertainties in the driving forces as well as from the
changing of the pollution pattern in-between the time span of
simulation. With the later growth factors get increasing influence
which are not implemented in the model sufficiently. Especially the
physiological mechanism of soil acidification and the direct nitrogen
uptake through the canopy are not implemented in process models yet,
because they are insufficiently known. Therefore these important growth
parameters must be evaluated using empirical risk factors.

The results of the simulation runs with FORSANA show positive effect
with respect to the growth of the stands. In general stemwood
production seems to benefit relatively more than total yield. Reduction
of the SO2-immissions
has a specific effect on the leaf area. As indicated already by the
analysis of the natural site characteristics the processing of the
scenarios also made clear the sensitivity of the stands to water
deficiency. It is shown that the already relatively low average
precipitation leads to a negative effect on the growth of the stands
independently of any pollution impact, because of drought stress. This
lead to the question of whether the competition of the ground
vegetation for water and nutrients could prevent the positive
development of the forest stands. Because of this combination of
enhanced growth rates and low precipitation the research area is quite
sensitive to potential changes in precipitation and therefore quite
vulnerable in the context of climate change.

Finally it is concluded, that the positive effects of immission
reductions until now could be proofed statistically as well as by the
simulations with the process based model. With respect to immission
reduction, results of the simulation runs show also a further positive
development. The decrease in pollution impact is opposed to some other
risks. From the simulation runs resulted an increased probability of
drought stress. Further risks, which are rising from the nitrogen
saturation of the soils and the soil acidification and affect future
stand development, are interpreted empirically from existing data,
because of lacking model implementation.